Patterned microwave susceptor

ABSTRACT

The heating effect of a microwave susceptor can be improved by providing a pattern of microwave transparent areas in the susceptor. The transparent areas are preferably circles having a diameter of about 0.5 inch. The distance between adjacent circles is preferably about 0.5 inch. The susceptor may be used to brown and crispen the crust of frozen pizza heated in a microwave oven. The crust of the pizza is browner, especially at its central area, than the crust of pizza heated using a conventional susceptor.

This invention is a microwave susceptor having a pattern of microwavetransparent areas that enhances the heating effect of the susceptor atits center.

BACKGROUND OF THE INVENTION

A microwave susceptor typically comprises a layer of metallized plasticfilm laminated to a dimensionally stable substrate, such as paperboard.The thickness of the metal is such that the metal absorbs microwaveenergy and converts it into heat. Such susceptors are commonly usedcommercially to brown and crispen food in contact with the susceptor.One example of such use is in connection with frozen, packaged pizzahaving a diameter of about 7 inches (about 18 cm). The susceptor, whichis placed under the pizza, browns and crispens the crust of the pizza.However, it has been found that a conventional susceptor does not brownor crispen the center of the pizza satisfactorily when the pizza has adiameter from about 8 to 12 inches (about 20 to 30 cm). U.S. Pat. No.4,896,009 to Pawlowski discloses that the browning and crisping effectof a susceptor used with pizzas having diameters between 7 and 12 inchescan be improved by providing one or more apertures at the center of thesusceptor. According to Pawlowski, the improvement is due to the escapeof vapor through the apertures, which allows the pizza to remain incontact with the susceptor. However, providing apertures in thesusceptor requires a separate step in the manufacture of the susceptorand produces chad that must be disposed of. It also destroys theintegrity of the susceptor, which forms part of the package for thepizza.

This invention provides a susceptor that produces results at least asgood as the results produced by the susceptor in Pawlowski by providinga pattern of microwave transparent areas in the susceptor. U.S. Pat.Nos. 4,883,936 and 5,220,143 disclose that the heating effect of asusceptor can be reduced in selected areas by providing a pattern ofmicrowave transparent areas in the susceptor, but the object of thisinvention is to increase, not reduce, the heating effect of thesusceptor. U.S. Pat. No. 5,530,231 discloses that the heating effect ofa susceptor can be increased by providing a pattern of microwavetransparent areas in the susceptor, but the patent fails to teach thepattern of this invention, which produces superior results.

SUMMARY OF THE INVENTION

This invention is an improvement in the typical microwave susceptorcomprising a layer of metallized plastic film laminated to adimensionally stable substrate, such as paper or paperboard. Thesusceptor of this invention has a pattern of substantially microwavetransparent areas in the layer of metal on the plastic film thatenhances the heating effect of the susceptor in the central area of thesusceptor.

Each transparent area is circumscribed, i.e., it is a closed geometricalfigure. Therefore, the susceptor in which the pattern is formed iselectrically continuous. The geometrical figure can be a polygon, suchas a triangle, rectangle or hexagon, a circle or elipse, a cross or astar. The geometrical figure preferably has an aspect ratio of fromabout 1 to 1 to 2 to 1. Accordingly, if the figure is a polygon, it ispreferably a regular polygon, such as a square. The figure is mostpreferably a circle.

The major linear dimension of the transparent area is between about 0.6and 2.5 cm. For example, if the area is a circle, the diameter of thecircle is from about 0.6 to 2.5 cm, and ideally is about 1.3 cm (about0.5 inch), which happens to be about ⅛ of the wavelength of microwavesin a conventional microwave oven. When the transparent area is a circleand the susceptor is used to brown the crust of a frozen pizza in amicrowave oven, a brown annular ring forms on the pizza around thecircle. The thickness of the annular ring (distance from the edge of thecircle to the edge of the browning) is about 0.13 inch (about 0.33 cm).When the diameter of the circle is more than about 0.5 inch (about 1.3cm), the thickness of the annular ring is about the same, but the areawithin the annular ring, which is not browned, is larger, so it is notdesirable to increase the diameter of the circle substantially aboveabout 0.5 inch (1.3 cm). When the diameter of the circle is less thanabout 0.5 inch (1.3 cm), less browning around the edge of the circle isobserved, e.g., the thickness of the annular ring is less, so it is notdesirable to decrease the diameter of the circle to less than about 0.5inch (1.3 cm).

The distance between adjacent transparent areas is preferably betweenabout one and three cm.

The transparent area can be formed in several different ways. Asdescribed in U.S. Pat. No. 5,530,231, a pattern of oil can be depositedon the plastic film before the metal is deposited on the film to preventthe deposition of metal on the film in the areas masked by the oil.Alternatively, an etchant, such as caustic solution, can be applied to ametallized plastic film to dissolve and wash away the metal to form thedesired transparent areas. The preferred technique, which is describedin U.S. Pat. No. 4,865,921, is to apply a chemical, such as sodiumhydroxide, to inactivate the metal, without removing it, in a pattern toform the desired transparent areas. Transparent areas can also be formedby cutting holes in the susceptor, as taught in the Pawlowski patentreferred to above, but since such structures are in the prior art, thisinvention is limited to susceptors that are imperforate.

The transparent areas are preferably concentrated at the center of thesusceptor since that is where improved browning is desired. Fewertransparent areas are needed as the distance from the center of thesusceptor increases. In the area within a radius of about two inches(about five cm) from the center, the proportion of the area of thetransparent areas to that central area of the susceptor (about 80 sq.cm) is preferably from about 10 to 20%. In the annular ring that extendsfrom about two inches (about five cm) to about four inches (about tencm) from the center of the susceptor, the proportion of the area of thetransparent areas to the total area of the susceptor is preferably fromabout 5 to 15%. The proportion of the area of the transparent areas tothe total area of the entire susceptor is preferably from about 7 to15%.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top view of the preferred embodiment of the improvedmicrowave susceptor of this invention.

FIG. 2 is a partial cross sectional view of the susceptor shown in FIG.1 taken along line 2—2.

FIG. 3 is a graph of the surface temperature of the central area of thecrust of a pizza heated in a microwave oven using the susceptor shown inFIG. 1 compared to the surface temperature of the central area of thecrust of a pizza heated in a microwave oven using a conventionalsusceptor.

FIG. 4 is a graph showing the degree of browning achieved using thesusceptor shown in FIG. 1 compared to the degree of browning achievedusing no susceptor and a conventional susceptor.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

As shown in FIGS. 1 and 2, a preferred embodiment of the improvedsusceptor comprises a layer of plastic film 10 on which is deposited,such as by vacuum deposition, a layer of metal 12, preferably aluminum.The thickness of the metal is such that it absorbs microwave radiationand converts the microwave energy into heat. The plastic film ispreferably made from polyethylene terephthalate and preferably has athickness of about 0.48 mil (about 12 microns). The metallized film islaminated to a layer of paperboard 14 using a conventional adhesive 16.

A pattern of forty-one circles 18 was formed in the metallized film byapplying a chemical, such as sodium hydroxide, to inactivate the metalin each circle. The inactivated metal is substantially transparent tomicrowave radiation. The diameter of each circle was about 0.50 inch(about 1.3 cm). The inactivating chemical was also used to form a gridpattern 20 in the annular peripheral margin 22 of the susceptor. Thewidth of the peripheral margin 22 was about 0.75 inch (about 1.9 cm).The overall width of the susceptor was 10.5 inches (about 27 cm) toaccommodate a pizza of about the same size (not shown) which is placedon top of the susceptor. The metal layer 12, which is visible as a graysubstrate beneath the clear plastic film 10, is indicated by stipplingin FIG. 1. The inactivated metal appears white.

A commercially available, frozen pizza conforming to the susceptor wasplaced on top of the susceptor and heated in a microwave oven. Luxtron™temperature probes were placed between the pizza and the susceptor inthe circle at the center of the susceptor and around the circle. Thisexperiment was repeated using a conventional susceptor, i.e., asusceptor in which the metal layer covered the entire surface of thesusceptor. The results are shown in FIG. 3, where line A represents theaverage temperatures recorded by the probes in contact with the circle,line B represents the average temperatures recorded by the probes incontact with the area around the circle, and line C represents theaverage temperature recorded by comparably placed probes using theconventional susceptor. As can be seen from FIG. 3, the susceptor ofthis invention produces a higher final temperature in the central areaof the pizza than a conventional susceptor.

The degree of browning of the crust of similarly heated pizza wasmeasured using a Minolta™ BC-10 bake meter, which measures bakingcontrast units (BCU). The lower the BCU, the browner the color.Measurements were taken at eight locations along a first diameter of thepizza and at eight other locations along a second diameter perpendicularto the first diameter. The results are shown in FIG. 4 for frozen pizzasheated using the susceptor shown in FIG. 1, a comparable conventionalsusceptor, and no susceptor, compared to the frozen pizza before beingheated. Line D represents the average BCU's recorded by the bake meterat all sixteen locations and line E represents the average BCU'srecorded by the bake meter at the ten locations closest to the center ofthe pizza. As can be seen from FIG. 4, pizza heated using the susceptorof this invention produces pizza that is browner overall than pizzaheated using a conventional susceptor, and that is especially browner atthe central area of pizza.

We claim:
 1. In an imperforate microwave susceptor comprising a layer ofmetallized plastic film laminated to a dimensionally stable substrate,the improvement wherein the susceptor has a pattern of substantiallytransparent circles in the layer of metal on the plastic film thatenhances the heating effect of the susceptor in the central area of thesusceptor, with each circle having a diameter of about 1.3 cm, thedistance between adjacent circles being between about one and three cm,and the susceptor being electrically continuous.
 2. The improvement ofclaim 1, wherein the proportion of the cumulative area of thetransparent circles in the central area of the susceptor to the totalarea of the central area of the susceptor is from about 10 to 20%, andthe central area of the susceptor having an area of about 80 sq. cm. 3.A microwave susceptor comprising: a dimensionally stable substrate; anda generally continuous layer of metallized plastic film laminated to thesubstrate to form a microwave susceptor, the film having spaced-apartmicrowave-transparent areas within one generally continuousmicrowave-interactive area for enhancing the heating effect of thesusceptor, each of the transparent areas having a major linear dimensionof about one eighth an operating wavelength of a microwave oven.
 4. Themicrowave susceptor of claim 3, wherein each of the transparent areashas a major linear dimension of about 1.3 cm.
 5. The microwave susceptorof claim 3, wherein each transparent area has a width to height ratiobetween about 1 to 1 and about 2 to
 1. 6. The microwave susceptor ofclaim 3, wherein each transparent area has a generally circular shape.7. The microwave susceptor of claim 3, wherein adjacent transparentareas are separated by between about 1 and 3 cm.
 8. The microwavesusceptor of claim 3, wherein the susceptor is electrically continuous.9. The microwave susceptor of claim 3, wherein the dimensionally stablesubstrate is paper or paperboard.
 10. The microwave susceptor of claim3, wherein the susceptor has a center portion surrounded by an outerportion, and wherein the center portion of the susceptor is at least 10cm from a nearest edge of the susceptor.
 11. The microwave susceptor ofclaim 10, wherein a cumulative area of the microwave-transparent areasdisposed within the center portion is about 10% to 20% of a total areaof the center portion.
 12. The microwave susceptor of claim 3, wherein acumulative area of the microwave transparent areas is about 7% to 15% atotal area of the susceptor.
 13. The microwave susceptor of claim 3,wherein the susceptor includes a peripheral margin portion having amicrowave-transparent grid pattern.
 14. The microwave susceptor of claim3, wherein the transparent areas become more concentrated toward acenter region of the susceptor.